Interpolation IIR filter for OFDM baseband processing

ABSTRACT

A filter for receiver and operative on a stream of OFDM symbols has a symbol timing identifier which indicates the time interval for each symbol and also indicates a non-truncation interval and a truncation interval of the stream of symbols. The stream of OFDM symbols is applied to an infinite impulse response (IIR) filter with a reset input for resetting internal registers such that during the non-truncation interval, the reset input is not asserted, and during the truncation interval of the stream of OFDM symbols, the reset input is asserted during the intervals between symbols, as identified by the symbol timing identifier. 
     A transmit filter for a stream of OFDM symbols, each symbol being separated into a first Tg interval, a second Tg interval, a symbol interval, and a final Tg interval, the filter has a stream modifier which discards the first Tg interval, accepts said second Tg interval, accepts the symbol interval and discards said final Tg interval, presenting to an infinite impulse response filter, in sequence, the second Tg interval, the symbol interval and the second Tg interval.

FIELD OF THE INVENTION

The present invention relates to filtering of data for use in an OFDMsignal processing system. More specifically, the invention relates tofiltering of OFDM baseband receive data and baseband transmit data usinga truncated and decimated IIR filter.

BACKGROUND OF THE INVENTION

FIG. 1 shows a typical wireless LAN/MAN system with a radio frequency(RF) unit 104 coupled to baseband processor 118. The receive pathincludes RF front end 106 which amplifies received signals, which arethen analog filtered 110 and digitized 112 at a sample rate Fs. Theincoming samples at rate Fs are decimated by filter 120 to generate adecimator output 124 at rate Fs/2. Transmit data 126 at rate Fs/2 isprovided to interpolation filter 122 which increases the incoming samplerate from Fs/2 126 to Fs, and those samples are applied to DAC 116,filtered 114, and applied to Tx Front End 108, which amplifies,upconverts, and couples to antenna 102. Typically the ADC and DACsampling rate (Fs) is an integer multiple of the sample rate of thebaseband processor generating transmit data 126 or receiving data 124,which simplifies the suppression of images caused by ADC 112 and DAC 116sampling. The analog filter 110 and 114 requirements are also relaxed asthese filters can be designed with a larger transition band. Theresidual image is filtered digitally in the baseband by theinterpolation 122 and decimation 120 filters.

In the receive path, the decimation filter 120 removes the residualimage and then down samples the input signal to the baseband samplingrate. In the transmit path, the interpolator 122 up-samples the basebandsignal by inserting zeros in alternate samples, which are then filteredin the upsampled signal to remove images.

Interpolation filter 122 and decimation filter 120 are typicallyimplemented in finite impulse response (FIR) filters, rather thaninfinite impulse response (IIR) filters in OFDM baseband processors. IIRfilters have a register configuration where computed terms are fed backto earlier registers, which results in greater hardware efficiency andthe need for fewer storage registers than FIR filters for the samespectrum shaping requirements. The drawback of IIR filters is theintroduction of inter-8 symbol interference (ISI), hence degrading theperformance of the wireless OFDM link in the presence of multipathreflection. FIG. 2A shows the affect of the increased filter impulseresponse on inter-symbol interference. A signal 202 represents the inputsignal at a first tap point of a multipath IIR filter, and signal 204represents the input signal at a subsequent filter tap point. The FIRfilter impulse response is shown in waveform 212, and the correspondingfilter tail time response 214 is shown at the same time resolution asincoming data, which includes symbol S1206 followed by cyclic prefix 208and symbol S2 210. As can be seen from S1 region 214 representing thepart of S1 which undesirably contributes to S2 filter output, a smallpart of the FIR response 212 from symbol S1 is bleeding symbol S2. Thefilter impulse response adds to the delay spread caused by multipath andthe overall spread can exceed the cyclic prefix 208. The part of theresulting delay spread that exceeds the cyclic prefix contributes toISI. FIG. 2B shows the much larger ISI effect of IIR filters, where theimpulse response 244 includes the much larger extent of S1 associatedwith the IIR filter tail extent 242 extending well into S1 which adds tothe S2 response. The filter tail 242 which extends into S1 for an IIRfilter as shown in FIG. 2B results in much greater ISI of S1 into S2than the FIR filter of FIG. 2A.

It is desired to utilize a filter for receive decimation and fortransmit interpolation, where the filter has a smaller number of tapssuch as an IIR filter, but without the excessive time response andrelated ISI associated with an IIR filter.

OBJECTS OF THE INVENTION

A first object of this invention is a receive decimation filter having aplurality of registers and operative on a stream of data which includessymbol data and final reflection data, the filter operative on thereflection data prior to the symbol data, where the filter is resetprior to the reflection data.

A second object of the invention is a transmit interpolation filterhaving a plurality of registers and operative on a stream of data whichincludes initial samples, symbol samples, and final samples, thetransmit interpolation filter discarding the initial samples, operativeon a cyclic prefix from memory followed by symbol samples, theinterpolation filter being reset prior to the application of the cyclicprefix to the interpolation filter.

A third object of the invention is a remapper for a transmitinterpolation filter which receives a plurality of samples, stores arange of samples from the input, and provides an output by re-orderingsamples accompanied by the previously stored range of samples.

A fourth object of the invention is an infinite impulse response filterwhich is reset prior to the arrival of conditioning data which isfollowed by symbol data, where the conditioning data for receive is acyclic prefix, and the conditioning data for transmit is remapped Tgsamples.

A fifth object of the invention is an infinite impulse response filterfor transmit filtering, where a stream of transmit data symbols, eachtransmit data symbol including in sequence a symbol, first gap data,second gap data is rearranged into a sequence of first gap data, secondgap data, and symbol, thereafter upsampled by insertion of 0 databetween samples, thereafter applied to an IIR filter which is reset atthe start of the application of the first gap data, thereby forming afiltered first gap data, filtered second gap data, and filtered symbol,where an output is formed by outputting and saving the filtered secondgap data, the filtered symbol, and then outputting the saved filteredsecond gap data, this sequence being done for each transmit data symbol.

SUMMARY OF THE INVENTION

A receive IIR filter with an impulse response equal to or longer than acyclic prefix of a preamble operates in a regular mode during which afilter reset is not asserted during a preamble part of a receivedpacket, the preamble part including a short and long preamble and asignal field. The IIR filter thereafter operates in a per-symboltruncated mode during a data part of a packet, the data part of thepacket including a plurality of data symbols, each preceded by a cyclicprefix, whereby the registers of the IIR filter are reset and releasedfrom reset prior to or during each cyclic prefix which precedes acorresponding data symbol.

A transmit filter includes a remapper for an IIR filter, where theremapper receives a succession of symbols in the time domain, such asfrom the output from an inverse FFT (IFFT) as a time domain sequencecomprising a plurality of complex values, each symbol of the successionbeing separated into a symbol interval, a first Tg interval, and asecond Tg interval. The remapper outputs, in sequence, the first Tginterval values, second Tg interval values, the symbol value, and thefirst Tg interval value again. The output of the remapper is provided toan upsampler for increasing the data rate such as by zero valueinsertion, and the upsampled values are provided to the input of an IIRfilter which is reset at the beginning of each remapped first Tginterval. The output of the IIR filter is directed to a controllerwhich, in sequence, discards the IIR filter output for the Tg1 intervaland replaces it with a stored IIR filter output corresponding to aprevious symbol first Tg1 interval, after which the IIR filter outputcorresponding to the filtered second Tg interval is output and alsostored into a temporary buffer, after which the filtered symbol isoutput, after which the filtered first gap Tg1 is output, followed bythe contents of the temporary buffer, which forms the cyclic prefixbetween the current symbol and the following symbol. A new transmitwindow is also defined by the filtered and stored second gap value,filtered symbol value, and second gap value which was previously stored.In this manner, a symbol value followed by a first gap value and secondgap value is converted into a filtered (and saved) second gap valuefollowed by a filtered symbol value, followed by a filtered first gapinterval, followed by the saved (filtered) second gap value, therebyallowing the use of an IIR transmit interpolation filter whileeliminating ISI from one symbol to the next. For each such symbol, theinternal registers forming the IIR filter are reset or initialized priorto the remapper outputting the second Tg interval to the IIR filter,thereby clearing any previous symbol value from the IIR filter prior tothe start of IIR filtering of the current symbol.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram for a receiver front end.

FIG. 2A shows a time diagram for a stream of signals and an FIR filterresponse tail.

FIG. 2B shows a time diagram for a stream of signals and an IIR filterresponse tail.

FIG. 3 shows a time diagram for inter-symbol interference with atruncated infinite response filter response during a symbol window.

FIG. 4 shows a time diagram for decimation with a truncated IIR filterover a packet having a preamble part and signal field part and a datapart.

FIG. 5 shows a time diagram for a truncated filter applied to an OFDMpacket.

FIG. 6 shows a block diagram for receive decimation filtering.

FIGS. 7A, 7B, and 7C show sample rearrangement for IIR cyclicinterpolation filtering of OFDM data.

FIG. 8A shows a block diagram for transmitter interpolation filteringincluding a remapper for the IFFT output.

FIG. 8B shows an example remapping of FFT output samples as performed bythe remapper of FIG. 8A.

FIG. 9 shows the time diagram for a sequence of samples applied to aninterpolation filter input.

FIG. 10 shows samples at the output of an interpolation filter.

FIG. 11 shows a block diagram for a filter suitable for use in thepresent invention.

FIG. 12 shows a detailed block diagram for one embodiment of a singleBiquad filter.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 3 shows a truncated IIR decimation filter for a receive decimatorfilter such as 120 of FIG. 1. The stream of input symbols 304 enters anIIR filter which has a non-truncation (normal) mode during which thefilter operates in a conventional manner with data clocking into aregister input and transferring to a register output on each clockcycle. The filter also has a truncation mode, where the registers arereset thus clearing all the information stored from previous samples,after which the registers of the filter operate on the newly arrivingdata and generate filter output values such as during non-truncationmode. In one example of the invention when receiving packets, thetruncation filter is an IIR filter with the intermediate value registersof the filter are reset at particular points inside the packet,typically at the start of the cyclic prefix of either a preceding orsuccessive data symbol.

In another example of the invention for reception of packets, the IIRfilter operates in a non-truncated mode during a packet preamble (forwhich filter introduced ISI is not a consideration), and in a truncatedmode when receiving a stream of symbols, such that the filter is resetfor each incoming symbol.

FIG. 3 shows a generalized stream 304 of data which includes symbols 304having a first symbol S1 322, a cyclical prefix CP 324, and a subsequentsymbol S2 326. Each symbol crosses a truncation boundary such as 312,when the internal registers of infinite impulse response IIR filter 308are taken out of a reset state. In one embodiment of the invention, areset signal is generated at the start of each cyclic prefix (CP) whichprecedes a symbol. In this manner, the IIR filter clocks in andinitializes using the CP which precedes the associated symbol. The IIRfilter then operates on the incoming symbol data until it is again resetwhen a new symbol window arrives, such as upon arrival of the next CP ofthe symbol stream. In another embodiment of the invention, thetruncation window 312 indicates the instant of IIR register reset, andthe window 312 may be placed at the start of the symbol window 316, suchthat the IIR filter is reset at the moment the symbol such as 326arrives at time 312, or the IIR filter may be reset during at aparticular instant which occurs during the interval of the cyclic prefix324. For symbol S2 of FIG. 3, first sample point A may be the firstsymbol data clocked into the filter after truncation boundary 312,followed by second at second sample point B, continuing until the end ofthe symbol window, which includes the subsequent CP 328. This truncationthrough reset boundaries which occur between symbols ensures thatsamples from the previous symbol do not add into the filter output forthe current symbol, thereby addressing ISI resulting from the longimpulse response time of the IIR filter. The advantage of this filtertruncation technique is that the distortion introduced as a result offilter truncation does not have as significant an impact as theinter-symbol interference (ISI) which is significantly reduced by thetruncation technique.

FIG. 4 shows a Wireless LAN (such as described in the IEEE wireless LANstandards IEEE 802.11a or IEEE 802.11g) OFDM packet 402 comprising ashort preamble 404, double cyclic prefix 406, long preamble 408, cyclicprefix 410, signal field 412, cyclic prefix 414, and a stream ofsubsequent data symbols 416, 420, each of which are separated by acyclic prefix 418, 422. As the packet enters the receiver, Automaticgain control (AGC) is performed over the initial short preamble samples404. The remaining samples of the short preamble 404 are used toidentify a WLAN packet and also to acquire a coarse estimate of thefrequency offset and symbol timing. The long preamble 408 is then usedto derive a finer estimate of the symbol timing and frequency offset.The signal field 412 is a BPSK modulated symbol that containsinformation such as the rate of transmission and the length of thepacket.

Truncated IIR filtering can be used on the subsequent stream of datasymbols 416, 420 once the symbol boundaries have been identified. Thestate of the IIR filter is then reset at the symbol boundaries, whichresults in removing the contribution to the impulse response from theadjacent symbol. Since this procedure requires knowledge of symbolboundaries, it can be performed only after the symbol timing has beenidentified from the long preamble 408.

FIG. 5 shows the position 522 in the WLAN packet 502 at whichnon-truncated (or regular) IIR filtering is switched to truncatedfiltering. The individual fields of the packet 502 are labeled asdescribed for FIG. 4. Regular filtering is performed over the longpreamble 508 and signal field 512. However the symbols in these fieldsare not affected by ISI. The long preamble 508 has a double guardinterval that minimizes ISI and the signal field 512 uses a very robustmodulation and coding that is tolerant to ISI. The parts of the packet502 which precede the first data symbol, including short preamble 504,long preamble 508, and signal field 512, as well as cyclic prefixes 506and 510, are referred to as a header part, and all header parts aresubject to regular (non-truncated) IIR filtering, whereas the IIR filterchanges to truncated mode filtering for each of the symbols 516 and 520of the data part of the packet which follows the header part. The symboltiming that was established during the long preamble 508 establishes theprecise symbol boundaries 516, 518, etc, and these boundaries are usedto hold the IIR filter in reset preceding each symbol 516, 518,releasing the IIR filter reset state as each symbol of the data part ofthe packet enters the filter.

FIG. 6 shows one example embodiment of the receive filter 600, suitablefor use as receive decimation filter 120 of FIG. 1. The input stream ofIQ data from the receive ADC 112 is applied to filter 602 and also tofine symbol timing 604, which locates the symbol boundaries based on thelong preamble part of the packet. Once the fine symbol-timing module 604establishes the symbol boundaries, it resets the IIR filter 602 state atthe first sample of the cyclic prefix of every symbol. Many differentconfigurations of IIR filters may be used 602, with each filterresetting at the beginning of each symbol. The filter is typically resetin its entirety (all of the registers forming the IIR filter are resetat once), or it may be partially reset, such as by clearing the contentsof the individual registers. Additionally, the filter may be reset at asingle point in time following the CP, as identified by a symbol timingfunction which determines the symbol timing of the regular data symbolsfrom the packet preamble, or the filter may be reset during an intervalof the CP prior to the arrival of a symbol.

A different form of the IIR can also be considered for use in thetransmit filter, shown as interpolation filter 122 of FIG. 1. A similarISI problem as was described for receive decimation filtering arises intransmit interpolation filtering when using IIR filters which have theadvantage of using fewer registers than FIR filters but the disadvantageof longer response time. The longer response time of the IIR filtercompared with the FIR filter can add signal energy from previoustransmit symbols into the current transmit symbol, which is asundesirable for the transmit case as for the previously describedreceive case. In transmit operation, it is possible to use the truncatedIIR filter for the transmit interpolation filter in a different mannerby using a remapper at the input of the IIR filter to rearrange gapvalues to the start of the symbol value, resetting the IIR registerswhen those gap values are presented to the input of the IIR filter, andusing an output controller at the output of the IIR filter fordiscarding the first part of the IIR filter output, storing andoutputting a valid part of the filtered output, then outputting thefiltered symbol value, and then outputting the previously stored output.This has the effect of resetting the filter to clear previous symbolvalues, and during the interval of time the transmit filter is“initializing” with data for the new symbol during a first gap interval,outputting the previous stored previous gap information for the previoussymbol. Then, during the current symbol interval, which starts after theIIR filter output is initialized with first gap data, outputtingfiltered first gap data followed by filtered symbol data, followed byfiltered first gap data which was previously stored.

FIG. 8A shows an example IIR transmit filter embodiment in a systemwhich is suitable for use in the transmitter of FIG. 1. The samples fora symbol are read out of the FFT output memory 802 such as the paralleloutput of a time domain sequence that is concatenated together for aparticular example shown in FIG. 8B. FFT 802 output consists of 64output values delivered in a parallel fashion to remapper 803 and shownas remapper input 840 sequence of FFT output values [0 . . . 63].Remapper 803 output 842 is shown for one example, where the symbolduration is divided into a symbol part such as [0 . . . 31], a first gappart such as [32 . . . 47] and a third gap part [48 . . . 63]. Theassociated symbol part S1, S2, S3 is placed after the associated firstand second gap TG1 and TG2 for each symbol. These are upsampled 804 byinserting the value 0 between samples, and applied to IIR filter 806input. The IIR filter is reset at the beginning of each symbol, and theIIR filter output for each time domain series input such as 32 . . . 47is noted as [32 . . . 47]′ to indicate the filtering operation on thesesamples, and shown as filter output 844. The controller 808 stores theseries of samples associated with the filtered second gap shown as [48 .. . 63]′ into a filter while simultaneously outputting them, such thatthe first 32 samples are discarded, and the next 32 samples are filteredand placed in front of the filtered symbol data [0 . . . 31]′ followedby first gap data [32 . . . 47], as shown in the sequence of remapperoutput 846. FIG. 9 shows a generalized version of the controller 808operation. The upsampler 804 of FIG. 8A inserts zeros between every twosamples that have been re-arranged by remapper 803 as was previouslydescribed, and the upsampler 804 hence doubles the sampling rate. Theupsampled signal is then applied to IIR filter 806 input as shown inFIG. 9. The FIR filter output is handled by the controller 808 asfollows:

1) The first Tg samples 1002 at the output of the filter are discardedto avoid samples with a truncated filter response;

2) The second Tg samples 1004 are output to the DAC and also stored in aseparate buffer 810 of FIG. 8;

3) The symbol 1006 is read out and output to the DAC;

4) The contents of the buffer 810 from step 4 are read out and output tothe DAC.

FIG. 7A shows the operation of the transmitter filter where the outputof the FFT 802 of FIG. 8A is coupled to the input of the remapper 803,which input is shown as parallel output 701 in FIG. 7A. The FFT output701 is a succession of parallel values (such as FFT[0] . . . FFT[63])which are provided during each symbol interval 705 and 707 shown. Theremapper 803 accepts the FFT output, which includes symbol S1 702 andGap 704, which gap is subdivided into Tg1 and Tg2, and rearranged andserialized with Tg1 inserted after the symbol part to form serial output703 to upsampler 804 which doubles the sample rate, such as by inserting0s into alternating samples. The remapper 803 output is formed byplacing the remapper input gap values 704 at the start of the serialstream 710 followed by symbol S1 712 which is formed by shorteningremapper input S1 702 by an interval equal to Tg2 as shown in stream703. The remapper output 703 thereby produces a serial stream of datacontaining Tg1, Tg2, S1, and Tg1. This serial stream of data isinterleaved with 0s by the upsampler 804 and is applied to the IIRfilter 806. At the beginning of each symbol frame interval Tsym, theassociated IIR filter 806 has all of its internal registers reset, asshown by waveform 730 asserting reset at the beginning of each Tsym. Theoutput of the IIR filter shown in FIG. 7B now contains a stream ofvalues, and since the IIR filter was reset 730 at the beginning of eachsymbol interval, the initial output 740 associated with interval Tg1contains invalid values, as the IIR filter started filtering from resetinternal register values until the passage of Tg1 interval and thepresentation of CP1 741 from filtered Tg2, followed by the filteredsymbol part 742 and the filtered first gap 743. The IIR filter outputfor the subsequent frame interval contains invalid output 744 followedby CP2 745 formed by the filtered Tg2 value from serial stream 703applied to the IIR filter, followed by filtered first gap values 747,followed by the invalid interval for the next symbol window. As is clearto one skilled in the art, the description of the output of the IIRfilter following each reset operation is a complicated mixture offeedback values formed from a discontinuous input value applied to resetIIR internal registers, however the duration of Tg1 is selected to bringthe filter output to a steady state value such that the distortionassociated with resetting the filter is minimized at the time of TG2 741output. The IIR controller 808 forms output values 812 using the outputstream from IIR filter 806 in combination with buffer 810, whichgenerates the controller output 812 shown in FIG. 7C. The value CP1 762associated with each current frame is output to serve as the cyclicprefix (CP) preceding the symbol, and the CP1 value is also saved into amemory, after which the present filtered symbol S1 764 from the IIRfilter is output, followed by the filtered first gap value 765, and thepreviously saved value 762 is output during the interval 766 whichfollows the corresponding current symbol S1 764. The subsequentcontroller output comprises CP2 768 formed from IIR filter output ofinput 745 (with contribution from TG1 of 714 to the filter input), andalso saved in buffer memory 810, followed by filtered S2 770 from filteroutput 746, followed by the filtered first gap value, followed by CP2772, which was saved in buffer 810 from CP2 768. The result of resettingthe filter, discarding the initial Tg1 interval output, saving CP1 andoutputting it at the end of the current symbol from the buffer has theeffect of shifting the symbol boundaries as shown in new S1 extent 748and 750.

One example embodiment of an IIR filter using BIQUAD filter elements isshown in FIG. 11. The number of BIQUAD sections required is N/2 where Nis the filter order, such that the example of FIG. 11 is a second orderfilter with identical processing for the I channel 1104, 1106, and the Qsection 1108, 1110. FIG. 12 shows an example BIQUAD filter section. Theregisters 1208, 1210 of each BIQUAD IIR filter section are reset usingsignal 1215 at the beginning of each symbol interval, thereby removingany previous symbol history from the present symbol to be filtered.

1. A filtering process for a stream of OFDM symbols applied to the inputof an infinite impulse response (IIR) filter, said stream of OFDMsymbols comprising a preamble interval followed by a plurality of dataintervals, each data interval having a cyclic prefix followed by a datasymbol, said IIR filter comprising a plurality of sequentially connectedregisters generating an output and having at least one sequentiallyconnected register output fed back to a preceding one of saidsequentially connected registers, said sequentially connected registersresponsive to a reset signal which initializes said sequentiallyconnected registers, the filtering process comprising the steps: apreamble step performed during said preamble interval where said resetsignal is not asserted; a data interval step performed during each saiddata interval where said reset signal is asserted during each of saidcyclic prefixes; and a step of removing a stream of filtered symbolsfrom said IIR filter output.
 2. The process of claim 1 where saidregister output fed back to the preceding one of said sequentiallyconnected registers includes multiplying said register output by acoefficient and feeding the multiplication result to said preceding oneof said sequentially connected registers.
 3. The process of claim 1where during said data interval step, said reset signal is asserted at abeginning or during a cyclic prefix.
 4. The process of claim 1 wheresaid reset signal of said preamble step and said data interval step isgenerated by a symbol timing element which identifies the boundaries ofeach data symbol of said data intervals.
 5. The process of claim 1 wheresaid IIR filter includes at least one BIQUAD filter.
 6. The process ofclaim 1 where said IIR filter has a response time which is greater thanthe duration of a cyclic prefix.
 7. A filter process for filtering astream of OFDM symbols applied to an infinite impulse response (IIR)filter, said stream of OFDM symbols comprising a plurality of packetdata, each packet data having a preamble interval followed by aplurality of data intervals, each data interval comprising a data symboland a cyclic prefix, said IIR filter formed from a plurality ofsequentially connected registers having at least one register output fedback to a preceding register of said sequentially connected registers,said sequentially connected registers responsive to a reset signal forresetting said sequentially connected registers, the filter processcomprising the steps: a register reset step, where during each cyclicprefix, said reset signal is asserted, thereby resetting thesequentially connected registers of said IIR filter; a filtering step,where during each data symbol and during each preamble interval, saidreset signal is not asserted; and a step for removing a stream offiltered symbols from the output of said IIR filter.
 8. The process ofclaim 7 where said reset signal is asserted once at the beginning orduring each of said cyclic prefixes.
 9. The process of claim 7 wheresaid IIR filter is a BIQUAD IIR filter.
 10. The process of claim 9wherein said BIQUAD IIR filter includes an I sample filter and a Qsample filter, each said sample filter comprising said sequentiallyconnected registers, each register of said sample filter is initializedby said reset signal, each said reset signal is asserted at thebeginning or during each of said cyclic prefixes.
 11. The process ofclaim 7 where said register reset step is performed for a short preambleand a long preamble part of data received from a wireless device. 12.The process of claim 7 where said stream of OFDM symbols is receivedfrom a device which transmits OFDM symbols according to IEEE 802.11a orIEEE 802.11g.
 13. The process of claim 7 where said IIR filter has aresponse time which is greater than the duration of a cyclic prefix.